An engine designed to be mounted on a large-size motorcycle has a large displacement per each cylinder, particularly in the case of a single or two cylinder engine. In such engines, a fluctuation in torque of a crankshaft due to an explosion load per each cylinder is great. In order to avoid applying an impact load due to the torque fluctuation to a transmission mechanism, it has been proposed to increase the mass of the flywheel connected to the crankshaft to suppress the rotation speed variation thereof.
Known connections, or couplings, between the flywheel and the crankshaft are, in general, constructed such that an outer peripheral surface of one end of the crankshaft is tapered so as to form a tapered shaft, an inner surface of a bore in the flywheel fitted on the tapered shaft is formed into a tapered surface, the crankshaft and the flywheel are wedge-engaged with each other by both the tapered surfaces, and the flywheel is fastened to the crankshaft by a bolt. As the tightening force on the bolt is increased, the flywheel and crankshaft will become more firmly wedged together. If the two are tightly wedged, then the flywheel will effectively absorb the above mentioned impact loads. However, because the tightening torque which can be applied to the bolt is limited, it imposes a restriction on the flywheel mass and/or engine size for which this approach is useful. In this connection, the greater the flywheel mass the higher the torque for tightening the flywheel is required. In practice, however, the tightening torque has a limit. In view of the above, another approach is used when a flywheel having a great mass is required. The flywheel is brought into spline engagement with the crankshaft. In this arrangement, however, some circumferential play exists between the flywheel and crankshaft due to a clearance between meshed teeth at the spline engagement. Consequently, torque fluctuation on the crankshaft imparts an impact load to each tooth at the spline engagement which develops high stress. This may result in the shortening of the torsional fatique life of the crankshaft. In addition, the spline clearance allows the tightened end face of the assembly to move slightly, and this is adequate to cause wear at that spot sufficient to reduce the tightening torque on the bolt.